The half-band width of a monochromator is defined as what?

The half-band width of a monochromator specifically refers to the range of wavelengths that are transmitted by the device at a point where the transmittance is 50% of its maximum value. This measurement is crucial in analytical chemistry because it provides an indication of the spectral resolution of the instrument; a smaller half-band width indicates higher resolution and specificity in measurements. Consequently, knowing the half-band width helps in understanding how well the monochromator can separate different wavelengths of light, which is essential for accurately analyzing the composition of various samples in clinical chemistry.

As for the other choices, while they address aspects related to optical properties and wavelength characteristics, they do not define the half-band width as precisely and are thus less relevant in this context. The focus on the specific percentage of maximum transmittance distinguishes the correct definition and underscores its importance in monochromator performance.